Restructure project

[brynpickering]

Summary of changes in this pull request:

• Moving to using pyproject.toml as is the recommended way to define python project metadata now.
• Move calliope into src to make sure tests are not getting confused about relative filepaths in a way we don't realise.
• Move from azure pipelines to github actions, which allow us to leverage the growing number of pre-built actions (like micromamba for speedy env install).
• Move tests out of the calliope package itself.
• Move and restructure requirements files so that dev install becomes a much easier one-liner for the mamba/conda part.

Reviewer checklist:

• Test(s) added to cover contribution
• Documentation updated
• Changelog updated
• Coverage maintained or improved

[codecov]

Codecov Report

Merging #488 (775dd6f) into main (cbed4c8) will decrease coverage by 3.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #488      +/-   ##
==========================================
- Coverage   94.98%   91.95%   -3.04%     
==========================================
  Files          31       31              
  Lines        3890     3890              
  Branches        0      923     +923     
==========================================
- Hits         3695     3577     -118     
  Misses        195      195              
- Partials        0      118     +118     

Files	Coverage Δ
src/calliope/__init__.py	100.00% <ø> (ø)
src/calliope/_version.py	100.00% <ø> (ø)
src/calliope/backend/backends.py	96.38% <ø> (ø)
src/calliope/backend/expression_parser.py	94.80% <ø> (ø)
src/calliope/backend/helper_functions.py	99.35% <ø> (ø)
src/calliope/backend/latex_backend.py	95.52% <ø> (ø)
src/calliope/backend/parsing.py	99.51% <ø> (ø)
src/calliope/backend/where_parser.py	98.43% <ø> (ø)
src/calliope/cli.py	81.67% <ø> (ø)
src/calliope/core/__init__.py	100.00% <ø> (ø)
... and 21 more

[brynpickering]

Hoping that codecov will recover and realise the error of its ways once this is merged into main and new PRs build on top of it (I am at 94% coverage locally, not sure why it is at 92% here.